It is known that when arrhythmias, such as ventricular fibrillation, occur this condition may be corrected by applying a high energy electrical current through the myocardium. Oftentimes this procedure is accomplished by means of chest paddles which are appropriately placed on the thorax of the patient, or by electrodes applied directly to the surface of the heart during open heart surgery. It is also known that ventricular defibrillation can be carried out by use of a temporary or a permanently implanted endocardial lead.
For example, one type of endocardial lead utilized for defibrillation includes two electrodes positioned on a single lead for applying a voltage to the myocardium for a fraction of a second. If the voltage is sufficient to depolarize the fibrillating portion of the myocardium, the heart is returned to a normal cardiac rhythm.
One of the advantages of an endocardial lead for performing a defibrillation procedure is that the energy requirements are substantially less than that required for external paddle defibrillators. One of the problems associated with present endocardial defibrillator leads is that with the high electrical energy which must be applied, there is a corresponding high current density established at the electrodes. With the high impedance associated with electrodes of relatively small surface area, it is difficult to deliver the required energy to the myocardium to provide effective defibrillation. If the electrodes are increased in size to thereby increase their surface area, the electrodes tend to form rigid segments in the defibrillator lead thereby reducing the flexibility of the lead.
In order for the defibrillator lead to be either temporarily or permanently implanted in a patient, the lead must be capable of withstanding repeated bending and torsional stresses. In addition, the electrodes must have relatively large surface areas in order to deliver the high levels of energy that are required for defibrillation. In addition, the defibrillator lead must also be biocompatible, as well as of a configuration such that the lead has a smooth outer surface in order to prevent damage to adjacent tissue when the lead is inserted or withdrawn from the venous system of the patient.